The present invention relates to an internal combustion engine, and particularly relates to the construction of the combustion chamber of a high compression type internal combustion engine.
As a method of improving specific fuel consumption, there is a method of increasing the compression ratio of an engine. However, in a conventional engine, when the compression ratio is increased and, in addition, normal gasoline is used, knocking occurs and, as a result, the exhaust emission and specific fuel consumption deteriorate. Consequently, in order to prevent knocking from occurring in such a conventional engine, it is necessary to use a gasoline having an octane number which is higher than that of normal gasoline. In order to increase the output power of an engine in such a way that the pressure in the combustion chamber peaks immediately after top dead center while suppressing the occurrence of knocking, it is necessary to increase the burning velocity. In a high compression type engine, if the pressure in the combustion chamber peaks immediately after top dead center by increasing the burning velocity while suppressing the occurrence of knocking, it is possible to approach the advancing limit of ignition timing, at which knocking occurs, to MBT (Minimum Spark Advance for Best Torque) even if normal gasoline having a low octane number is used. As a result, a satisfactorily high output can be obtained and, at the same time, the exhaust emission and specific fuel consumption can be improved over those of the prior art even if gasoline having a low octane is used. However, in a conventional engine, when the compression ratio is increased and, in addition, normal gasoline is used, since knocking occurs as mentioned above, the exhaust emission and a specific fuel consumption deteriorate.